Modern electronic communication devices, such as devices configured to communicate over transmission media such as optical fibers, electronic wires/cables, or wireless links, all operate by modulating signals and sending these signals over the applicable transmission medium. These signals, which generally travel at or near the speed of light, can be subjected to various types of degradation or channel impairments. For example, echo signals can potentially be generated by optical fiber or wire/cable mediums whenever the modulated signal encounters junctions in the optical fiber or wire/cable. Echo signals can also potentially be generated when wireless signals bounce off of wireless reflecting surfaces, such as the sides of buildings, and other structures. Similarly, frequency shifts can occur when the optical fiber or wire/cable pass through different regions of fiber or cable with somewhat different signal propagating properties or different ambient temperatures. For wireless signals, signals transmitted to or from a moving vehicle can encounter Doppler effects that also result in frequency shifts. Additionally, the underlying equipment (i.e. transmitters and receivers) themselves do not always operate perfectly, and can produce frequency shifts as well.
These echo effects and frequency shifts are undesirable and, if such shifts become too large, may adversely affect network performance by effectively lowering maximum attainable data rates and/or increasing error rates. Such performance degradation is particularly problematic in wireless networks, which are straining to accommodate more and more users, each desiring to send and receive ever-increasing amounts of data. Within wireless networks, the adverse effects arising from echo effects and frequency shifts stem at least in part from the characteristics of existing wireless devices having wireless communication capability. In particular, these portable wireless devices (such as cell phones, portable computers, and the like) are often powered by small batteries, and the users of such devices typically expect to operate them for many hours before recharging is required. To meet these user expectations, the wireless transmitters on these devices must output wireless signals using very small amounts of power, making it difficult to distinguish the wireless radio signal over background noise.
An additional problem is that many of these devices are carried on moving vehicles, such as automobiles. This causes additional complications because the low-power wireless signal transmitted by these devices can also be subjected to various distortions, such as varying and unpredictable Doppler shifts, and unpredictable multi-path effects often caused by varying radio reflections off of buildings or other structures.
Moreover, the noise background of the various wireless channels becomes ever higher as noise-producing electrical devices proliferate. The proliferation of other wireless devices also adds to the background noise.